"My car is making me fat" sounds about as plausible as "My dog ate my homework." But don't laugh. The truth is that the lack of physical activity in our daily lives is a huge contributor to obesity. And what contributes more to inactivity than our dependence on the automobile?

A recent study of the link between driving and obesity showed a result that surprised even its lead researcher: Every 30 minutes you spend each day in a car increases your risk of being obese by 3%.

"Three percent for a half-hour is a heck of a result," says Lawrence D. Frank, PhD, who led the Georgia Institute of Technology study involving nearly 11,000 Atlanta-area residents.

The study also showed that:

90% of participants reported not walking at all. The average person in the study spent one hour or more per day in a car (driving or riding). Some spent more than five hours.

People who lived in neighborhoods with shops and offices within walking distance were 35% less likely to be obese than people who lived in sprawling, residential-only suburbs.

An average white male (height 5'10") living in a compact community with nearby shops and services weighed 10 pounds less than a similar white male living in a low-density subdivision.

Three out of every four people using mass transit had to walk to or from a stop, and were likely to get the surgeon general's recommended 30 minutes per day of physical activity.

For the average study participant, each kilometer walked (that's just over a half mile) per day translated into an almost 5% reduction in the probability of being obese.